For a demodulating apparatus that demodulates a digitally modulated signal, the output channels of its tuner may or may not be connected with the input channels of downstream devices on a homopolar basis (i.e., between the same polarities) or between the same channels, illustratively in consideration of the ease of wiring between the tuner that receives the signal and the downstream devices such as A/D converters; there may well be cases where the connections are made between different polarities (i.e., on a heteropolar basis) or between different channels. In such cases, one or both of the I-channel (in-phase component) and Q-channel (orthogonal component) may be inverted in polarity, or the I-channel and Q-channel may be switched over when the input signal is admitted to the demodulating apparatus.
FIGS. 1 through 6 show patterns of connection between a tuner 11 and downstream A/D (Analog/Digital) converters 12-1 and 12-2, as well as the phase relationship between the I-channel and Q-channel of the input signal being admitted to a demodulating apparatus 13 in each connection pattern.
In the connection pattern of FIG. 1, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a homopolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a homopolar basis; an I-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a homopolar basis; and a Q-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a homopolar basis. Thus as shown on the right-hand side in FIG. 1, the I-channel signal is input to the demodulating apparatus 13 as the in-phase component and the Q-channel signal in input as the orthogonal component. That is, the I-channel and Q-channel of the input signal are input to the demodulating apparatus 13 in homopolar and in-phase fashion. In the description that follows, the connection pattern of FIG. 1 may be referred to as the basis pattern.
In the connection pattern of FIG. 2, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a homopolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a homopolar basis; a Q-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a homopolar basis; and an I-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a homopolar basis. Thus as shown on the right-hand side in FIG. 2, the I-channel signal is input to the demodulating apparatus 13 as the orthogonal component and the Q-channel signal is input as the in-phase component. That is, as compared with the basic pattern, the I-channel and Q-channel of the input signal are inverted in phase when input to the demodulating apparatus 13. In the description that follows, the inversion of the I-channel and Q-channel in phase may be referred to as the inversion of the I-channel and Q-channel or simply as the inversion of the channels.
In the connection pattern of FIG. 3, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a heteropolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a homopolar basis; an I-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a heteropolar basis; and a Q-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a homopolar basis. Thus as shown on the right-hand side in FIG. 3, the I-channel and Q-channel of the input signal are each offset by +π/2 radians when input to the demodulating apparatus 13, compared with the connection pattern of FIG. 2.
In the connection pattern of FIG. 4, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a homopolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a heteropolar basis; an I-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a homopolar basis; and a Q-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a heteropolar basis. Thus as shown on the right-hand side in FIG. 4, the I-channel and Q-channel of the input signal are each offset by −π/2 radians when input to the demodulating apparatus 13, compared with the connection pattern of FIG. 2.
In the connection pattern of FIG. 5, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a homopolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a heteropolar basis; a Q-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a heteropolar basis; and an I-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a homopolar basis. Thus as shown on the right-hand side in FIG. 5, the I-channel and Q-channel of the input signal are each offset by −π/2 radians when input to the demodulating apparatus 13, compared with the basic pattern.
In the connection pattern of FIG. 6, the I-channel output terminals of the tuner 11 are connected to the A/D converter 12-2 on a heteropolar basis; the Q-channel output terminals of the tuner 11 are connected to the A/D converter 12-1 on a homopolar basis; a Q-channel signal having undergone A/D conversion is input to the I-channel input terminal of the demodulating apparatus 13 on a homopolar basis; and an I-channel signal having undergone A/D conversion is input to the Q-channel input terminal of the demodulating apparatus 13 on a heteropolar basis. Thus as shown on the right-hand side in FIG. 6, the I-channel and Q-channel of the input signal are each offset by +π/2 radians when input to the demodulating apparatus 13, compared with the basic pattern.
In the connection patterns of FIGS. 5 and 6, there exist differences only in the amount of phase rotation in the input signal admitted to the demodulating apparatus 13 when compared with the basic pattern; the inversion of the channels does not occur. Thus the input signal can be correctly demodulated by suitably carrying out phase control.
In the connection patterns of FIGS. 2 through 4, on the other hand, the channels of the input signal admitted to the demodulating apparatus 13 are inverted, so that the input signal cannot be demodulated correctly through phase control alone.
Furthermore, the up-conversion or down-conversion performed upon transmission/reception may cause the inversion of one or both of the I-channel and Q-channel of the signals output from the tuner 11.
Conventionally, under these circumstances, the polarities of the I-channel and Q-channel of the signals output from the tuner 11 were detected beforehand and the wiring between the tuner 11 and the A/D converters 12-1 and 12-2 was determined accordingly, which prevented the inversion of the channels of the input signal admitted to the demodulating apparatus 13. Alternatively, the inversion of the channels of the input signal to the demodulating apparatus 13 was detected beforehand based on the polarities of the I-channel and Q-channel of the signals output from the tuner 11 as well as on the wiring between the tuner 11 and the A/D converters 12-1 and 12-2, and the external settings of the demodulating apparatus 13 were changed accordingly, which allowed the channels of the input signal to be switched.
However, there are numerous types of tuners and their characteristics are different from one type to another. Furthermore, there may be a plurality of common carriers between points of transmission and reception within a communication system, so that it can be difficult to detect the presence or absence of the inversion of the channels in polarity due to the up-conversion and down-conversion involved. In such cases, conventionally the user of the demodulating apparatus 13 may monitor the input signal from an upstream processing system and, while verifying the result of synchronism acquisition by the demodulating apparatus 13, may change the external settings of the demodulating apparatus 13 to switch the channels of the input signal as needed. This, however, can be a very troublesome procedure for the user.
In order to overcome that trouble, techniques have been proposed whereby one of the I-channel and Q-channel of the input signal is automatically inverted in polarity if the channel polarities of the input signal are found inverted (e.g., see Patent Document 1).
FIG. 7 shows an example in which the invention described in the above-cited Patent Document 1 is applied to the demodulating apparatus 13 indicated in FIGS. 1 through 6. The demodulating apparatus 13 in FIG. 7 is made up of a channel control circuit 41 that controls the channel polarities of the input signal, a frame synchronization circuit 42 that provides frame synchronization control of the input signal, a phase synchronization circuit 43 that performs phase synchronization control of the input signal, and an error correction and decoding circuit 44 that executes error correction and decoding of the input signal.
For example, error correction and decoding are carried out in the demodulation process notably under the DVB-S.2 standard, a standard transmission specification for satellite broadcasting. In the demodulating apparatus 13, the channel control circuit 41 monitors the status of frame synchronization during Viterbi decoding by the error correction and decoding circuit 44. If an asynchronous state is detected for a predetermined time period, the channel control circuit 41 adjusts the phases of the I-channel and Q-channel in units of π/4 or inverts the polarity of the I-channel alone. This makes it possible automatically to invert the I-channel of the input signal in polarity for correct demodulation even where the channels of the input signal are found inverted.
Patent Document 1: Japanese Patent Laid-Open No. Hei 5-160863